Regulation of lipid metabolism by water temperature and photoperiod in yellowtail Seriola quinqueradiata

To improve the production efficiency of yellowtail Seriola quinqueradiata aquaculture, we measured changes in the activities of two lipid metabolism-related enzymes, carnitine palmitoyltransferase 2 (CPT2) and glucose-6-phosphate dehydrogenase (G6PDH), in the dark muscle and livers of 0- and 1 year-old fish over the entire culture period. Concomitantly we also investigated two factors that regulate these enzymes, namely, water temperature and daylength, under natural and controlled light conditions. In 0 year-old fish, high CPT2 activity was observed with increased water temperature/longer daylength, whereas high G6PDH activity was observed with decreased water temperature/shorter daylength. The activities of these enzymes were inversely correlated with each other. The changes in CPT2 and G6PDH activities were similar in 1- and 0 year old fish cultured with and without light control. To determine the major regulatory factors of CPT2 and G6PDH activities, two experiments were performed, starting on the vernal and autumnal equinox days, respectively. Fish were reared under natural and controlled light conditions (light:dark, 12:12 h) in both experiments. CPT2 activity did not differ according to light conditions or water temperature. G6PDH activity did not differ according to light conditions, but an increase in G6PDH activity was confirmed upon lowering of the water temperature. In summary, the activities of these two lipid metabolism-related enzymes changed seasonally and the main regulating factor may be water temperature. These results provide information for determining the appropriate lipid level and fatty acid composition of the seasonal diet for cultured yellowtail.